CN1378581A

CN1378581A - Hydrophobic material

Info

Publication number: CN1378581A
Application number: CN00813986A
Authority: CN
Inventors: 阿什利·W·琼斯; 罗伯特·N·拉姆; 张华�
Original assignee: Unisearch Ltd
Current assignee: Unisearch Ltd
Priority date: 1999-08-20
Filing date: 2000-08-21
Publication date: 2002-11-06
Anticipated expiration: 2020-08-21
Also published as: KR20020040782A; CN1203152C; WO2001014497A1; EP1210396B1; CA2383234A1; AUPQ234599A0; NZ517308A; US6743467B1; JP2003507567A; DE60031692D1; EP1210396A4; HK1047599B; ATE344308T1; HK1047599A1; EP1210396A1

Abstract

The present invention provides a method of forming a modified gel capable of forming a hydrophobic surface on which water has a contact angle of at least 150 DEG . The invention also provides a method of forming a hydrophobic coating from the modified gel. In some embodiments, the hydrophobic coating has an extremely high hydrophobicity with a contact angle in excess of 165 DEG , and good substrate adhesion. The method involves bonding a gel with particulate matter. It is believed that the chemical hydrophobicity of the gel is enhanced by the physical roughness of the particulate matter.

Description

Hydrophobic material

Technical Field

The present invention relates to coating technology. In particular, the present invention relates to hydrophobic coating materials and methods of forming coatings from these materials.

Background

Hydrophobic coatings are water-repellent coatings that are directly applied to reduce icing and fouling of other surfaces. Such coatings can also protect the surface from attack by water-insoluble electrolytes such as acids and bases and microorganisms.

In the past, surface encrustation, corrosion, ice formation and fouling were prevented by coatings containing polymeric films, hydrophobic solid fillers and hydrophobic liquids. One of the disadvantages of using such coatings is that they do not achieve multi-purpose protection, as they are often not versatile enough to resist damage from a variety of causes.

It is well recognized that the wettability of various substances depends on both the physical and chemical heterogeneity of the material. It has been well established for a long time that the contact angle θ of a drop of liquid on the surface of a solid substance is used as a quantitative test for the wettability of a particular solid. If the liquid is completely dispersed on the surface to form a film, the contact angle θ is 0 and if the bead on the surface of the substance is at an angle, the surface is considered non-wetting. For water, if the contact angle is greater than 90 °, the surface of the material is generally considered to be hydrophobic.

Examples of materials where the droplet has a large contact angle include water on wax, which has a contact angle of about 107 °. A hydrophobic coating having a contact angle of at least 150 °, preferably at least 165 °, is desirable in many applications.

A "gel" is a substance that contains a continuous solid framework surrounding a continuous liquid phase. The liquid prevents the solid from collapsing, while the solid prevents the liquid from escaping. The solid skeleton can be formed by joining colloidal particles together.

The present inventors have now developed a method of preparing a material which, when coated on a surface, is capable of imparting hydrophobicity to the surface.

Brief description of the invention

In a first aspect, the present invention provides a method of preparing a material capable of being applied to a surface, the method comprising the steps of:

(a) providing a precursor capable of reacting to form a gel,

(b) the precursors are reacted together to form a gel,

(c) adding particulate matter to the gel to form a mixture, the particulate matter being capable of chemically binding to the gel, and

(d) treating the mixture to form a modified gel, wherein the particulate matter is bound to the gel and the modified gel is capable of forming a hydrophobic surface having a water contact angle of at least 150 °.

In a second aspect, the present invention provides a method of forming a coating on a substrate, the method comprising steps (a) to (d) of the first aspect of the invention, and further comprising the steps of:

(e) coating the modified gel onto a substrate, and

(f) treating the coated modified gel to form a coating on the substrate, the coating having a hydrophobic surface with a water contact angle of at least 150 °.

Preferably, a "hydrophobic surface" as defined in the first or second aspect of the invention is a surface on which the water contact angle is at least 155 °. More preferably, the contact angle is at least 160 °. Even more preferably, the contact angle is at least 165 °.

The hydrophobicity of the hydrophobic material when applied to a surface is preferably due to both the chemical nature of the modified gel and the physical roughness of the material. It is envisaged that the modified gel of the first aspect of the invention may be used with solid materials in a variety of forms, including bulk materials, thick coatings and films.

The gel acts as a cross-linking agent to bind the particulate material and binds the modified gel to the desired substrate. Any known method of forming a gel may be used. Typically, the precursor defined in step (a) in the first and second aspects of the invention comprises at least water, a solvent and a metal alkoxide such as one of the following:

tetramethoxysilane (abbreviated as TMOS), Si (OCH)₃)₄；

Tetraethoxysilane (abbreviated to TEOS), Si (OCH)₂CH₃)₄；

Titanium tetraisopropoxide, Ti (O-iso-C3H7)₄；

Titanium tetramethoxide, Ti (OCH)₃)₄；

Tetraethoxytitanium, Si (OCH)₂CH₃)₄；

Titanium tetrabutoxide, Ti [ O (CH)₂)₃CH₃]₄

Zirconium n-butoxide, Zr (O-n-C)₄H₉)₄。

The solvent may comprise an alcohol such as methanol, ethanol, isopropanol or butanol. In addition, the solvent may further contain hexane or diethyl ether.

For example, silicate gels can be synthesized by hydrolyzing an alkoxide dissolved in an alcohol with an inorganic acid or an inorganic base or an organic acid or an organic base. The final product is a silica network.

Step (b) of reacting the precursors together in the first and second aspects of the invention may be carried out by refluxing the precursors for an extended period of time, for example 4 to 24 hours.

The particulate material defined in step (c) of the first and second aspects of the invention may consist of particles of substantially the same diameter, or of a series of particles of different diameters. Preferably, at least some of the particles have a diameter of from 1nm to 500. mu.m. More preferably, the range is 1nm to 100. mu.m. Even more preferably, the range is 1nm to 1 μm. Even more preferably, the range is from 1nm to 100nm, even more preferably, the range is from 5nm to 50 nm. In one embodiment, the particulate matter consists of particles having a diameter in the range of 1nm to 500 μm. In another embodiment, the major particle diameter of the particulate component is from 5nm to 50 nm. In yet another embodiment, the average particle size is from 5nm to 20 nm. In yet another embodiment, the average particle size is about 15 nm.

Step (d) of the first and second aspects of the invention may be carried out by first thoroughly mixing the mixture in, for example, an ultrasonic bath. Optionally, an alcohol such as isopropanol may be added to the mixture during this step to assist in the dispersion of the particulate material. Next, the mixture is refluxed to facilitate the formation of a chemical bond between the particulate matter and the gel.

Step (e) of coating the modified gel onto the substrate may be carried out by any known technique for forming a coating from a liquid, such as spin coating, dip coating or spray coating.

Step (f) may comprise drying the applied modified gel until a solid coating is formed. There may be solvent that needs to be removed from the modified gel, and if so, drying may include heating the coated modified gel to a temperature at least sufficient to evaporate the solvent. It will be appreciated that the drying temperature will depend on the melting point of the matrix and the type of gel. The drying time of the particle coating is generally dependent on the temperature used and to some extent on the thickness of the coating. For silica coatings, it has been found that heating temperatures of 120 ℃ to 400 ℃ for 10 to 30 minutes are suitable if the substrate is able to withstand. When the melting point of the matrix is low, vacuum drying or vacuum drying in combination with heating is preferred.

The elasticity and flexibility of the coating may be enhanced by adding a polymer component to the gel in step (c). Alternatively, the polymer component may be added directly to the gel before or after step (c). The polymer component is preferably bound to the gel and in step (d) is bound to the particulate material, preferably being hydrophobic, or rendered hydrophobic by step (d).

When the polymer component is incorporated into the gel, the method of preparation should also include the step of adding a surface modifier to the gel to increase the inherent chemical hydrophobicity of the hydrophobic surface prior to step (d). Additionally, the surface modifying agent may enhance the bonding between the gel and the particulate matter. The surface modifying agent may be a compound containing one or more condensation curing groups (condensation cure groups) and one or more hydrophobic groups. One isThe one or more condensation cure groups may include one or more of the following groups: acetoxy, enoyloxy (enoxy), oxime, alkoxy or amine. The surface-modifying agent may also comprise SiR (OAc)₃Wherein R is a hydrophobic group such as methyl, ethyl, vinyl, or trifluoropropyl, and Ac is acetyl. In a preferred embodiment, the particulate material comprises silica, the gel comprises a silicate gel, and the surface modifier comprises methyltriacetoxysilane. The step of adding the surface modifier may be performed just before step (c), during step (c), or immediately after step (c).

The gel, particulate material, and optional polymer components preferably form a slurry when mixed and are reacted together in step (d) of the first and second aspects of the invention.

In a preferred embodiment, the particulate material consists of flame-hydrolysed silica powder and the gel precursor contains a compound capable of forming a silica gel, such as TMOS or TEOS. A suitable polymer component in this case is Polydimethylsiloxane (PDMS), each chain bearing a hydroxyl-terminated polymer at the end. The resulting modified gel consists of silica particles chemically bonded to a siloxane network and surrounded by a liquid.

Flame-hydrolyzed silica powders are relatively inexpensive and are available as Aerosil from Degussa limited^TMSilica powder having a primary particle size of 5 to 50 nm. Although the flame-hydrolysed silica particles are initially hydrophilic, the surface chemistry is altered in step (d) by converting the silanol functions (≡ Si-OH) on the particle surface to siloxane bonds (≡ Si-O-Si ≡). This interaction imparts the advantage of hydrophobicity to the particles through reaction with PDMS or surface modifiers. The modified gels prepared by this method are particularly hydrophobic for two reasons. First, the chemical nature of the siloxane bond renders itself hydrophobic. Second, in flame-hydrolyzed silica, the small particles provide the modified gel with a small order of magnitude of roughness, which increases the hydrophobicity of the silica.

Although silica and silica-based particles are preferred, other substances that are hydrophobic, which may be made from very small particles, may be used in conjunction with the hydrophobic gel that is capable of binding to the particles. Examples thereof include particles and gels formed of metal oxides, such as titanium dioxide. By way of example, the titanium dioxide particles may be reacted with a titanium dioxide gel formed from titanium tetraisopropoxide.

Now, specific embodiments in the first and second aspects of the present invention are described. In step (a), the precursor was 5g TEOS, 1.7g water acidified with HCl to a pH of about 4, and 2.7g ethanol. In step (b), the mixture is refluxed for 6 to 36 hours to form a silica gel. In step (c), the particulate material added to the gel is 0.5g to 2.5g of flame-hydrolysed silica powder to form a mixture. Then, about 0.5g to 5g PDMS and about 50g isopropanol were added to the mixture. In step (d), the reactants are thoroughly mixed and shaken in an ultrasonic bath for 15-30 minutes to form a homogeneous slurry. The slurry was then refluxed for an additional 6-24 hours to form a silica gel chemically bonded to the silica powder and PDMS. The slurry is a modified gel that can be used to form a coating according to the second aspect of the invention. In a preferred embodiment, step (e) comprises depositing the slurry droplets on the substrate during rotation of the substrate. Typically the substrates are metals, glasses and ceramics. The speed of substrate rotation was 1000-. In step (f), the coated substrate is placed in an oven at a temperature of about 400 ℃ for 10-30 minutes.

In a third aspect, the present invention provides a modified gel prepared according to the method of the firstaspect of the invention.

In a third aspect, the present invention provides an object having a surface, wherein at least part of said surface is coated with a hydrophobic coating formed from a modified gel prepared according to the method of the first aspect of the present invention.

In a fifth aspect the present invention provides a hydrophobic coating prepared according to the method of the second aspect of the invention.

In a sixth aspect, the present invention provides an object having a surface, wherein at least part of said surface is coated with a hydrophobic coating prepared according to the method of the second aspect of the present invention.

In a seventh aspect, the present invention provides a use of the method of the second aspect of the invention for coating at least a portion of a surface of an object.

Surfaces that may be treated with hydrophobic coatings include metals, alloys, glasses, ceramics, composites, but may also include other materials. Surface treatment may be used to prevent corrosion or to form crystalline centers. The treatment may be used to prevent icing of surfaces, to form anti-binding hydrophobic coatings for above ground fixtures such as buildings and other structures, to provide anti-icing and anti-corrosion coatings for aircraft, or to provide anti-icing, anti-fouling and anti-corrosion coatings for submarines and inland waterway vehicles. An advantage of a hydrophobic anti-fouling coating formed from silica is that it is less toxic to the marine environment than most currently applied coatings.

Other applications include increasing roof resistance to the growth of micro-vegetation. The hydrophobic coating may also be used to extend the life, performance and reliability of devices and equipment.

Other applications contemplated include reducing friction in watercraft such as canoes, yachts, boats, and other watercraft, improving corrosion protection of cooling systems in internal combustion engines using closed heat exchangers (containing liquid heat exchangers), providing corrosion protection and anti-icing coatings for machine chassis such as tractors and combines, and general purpose agricultural machinery.

The hydrophobic coating according to the present invention can also be used to waterproof building foundations and structures and radiant waste storage devices, extend the operational life of water cooling towers, protect railroad ties against microscopic plants, provide anti-icing coatings for cooling rooms, refrigerators and cooling units, extend the life of hydroelectric dam installations, and improve the efficiency of wind driven motors. The metal parts coated in the hydrophobic coating are less prone to rust, since water will be driven off the metal surface. The hydrophobic coating may also be used on windows, such as the windshield of an automobile, if it is very transparent to visible light.

In this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

In order that the invention may be more clearly understood, reference will now be made to the following description taken in conjunction with the accompanying drawings.

Brief description of the drawings

FIG. 1 is a photograph of a hydrophobic coating of silica taken by a scanning electron microscope at 5000 Xmagnification;

figure 2 is a scanning electron micrograph (x 100000) at greater magnification of the same hydrophobic coating depicted in figure 1.

Modes for carrying out the invention

A method of preparing a silica-based hydrophobic coating will now be described. At this time, two basic chemical reactions of the sol method are:

(1) hydrolysis (ii) a And

(2) condensation of (alcohol formation) and

(Water is produced).

The hydrolysis and condensation reactions are generally simultaneous.

The precursor used to prepare the gel was tetraethoxysilane (Si (OCH)₂CH₃)₄) The particulate material isContaining hydrolysed silica powder (Aerosil)^TMSilica). The silica powder has a primary particle size of 5-50nm and a specific surface area of about 50-600m²(ii) in terms of/g. Since a large specific surface area affects the surface roughness of the coating and is thus related to the hydrophobicity of the coating, it is an important feature of silica.

The polymer component is Polydimethylsiloxane (PDMS):

PDMS is an unusual heat-resistant polymer that can withstand temperatures up to about 400 ℃ without deforming. It also has a high electrical resistance, is able to withstand outdoor UV radiation with little or no deformation or degradation for at least 10 years. PDMS is terminated with hydroxyl groups that serve as siteswhere the PDMS chains can react and bond with the silica gel or silica particles. When the hydroxyl groups are removed, the PDMS becomes hydrophobic. The viscosity of the PDMS used in this embodiment is approximately 90-20,000 cSt.

The modified gel is preferably formed as a slurry that can then be applied to a substrate to form a coating. Known techniques for forming conventional gel coats can be used. One technique is to coat the slurry onto the substrate, but the uniformity of the resulting coating is varied. The second technique is dip coating. The substrate is immersed in the slurry and pulled out at a specific rate. The third technique is spin coating. In this technique, the substrate is attached to a horizontal platform capable of rotating at high speed, for example, several thousand rpm. The slurry is deposited as droplets onto a substrate that is simultaneously rotating, resulting in a highly uniform coating. The coating thickness can be controlled by adjusting the rotation rate or the volume of the precipitated slurry. Another coating method is spray coating. The slurry spray solution is sprayed onto the substrate at a suitable distance, flow rate and instant to form a uniform coating of suitable thickness.

Example 1

The hydrophobic silica coating was prepared as follows:

● the following ingredients were mixed together:

5g TEOS；

1.7g of water acidified with HCl to a pH of about 4; and

2.7g of ethanol.

The mixture was refluxed for about 6-36 hours to form a silica gel.

● the following ingredients were added to the mixture:

0.5-5g PDMS with a viscosity of about 90-150 cSt;

0.5-2.5g of Aerosil from Degussa Limited^TM200 (flame-hydrolyzed silica powder, average particle size 15 nm); and

about 50g of isopropanol.

● the reactants were mixed thoroughly and shaken in an ultrasonic bath for 15-30 minutes to disperse the particles and form a homogeneous slurry. A typical vibration frequency of about 40KHz is sufficient.

● the reaction was removed from the ultrasonic bath and refluxed for approximately 6 hours. The slurry now obtained is suitable for preparing hydrophobic coatings.

● the slurry is deposited on the substrate surface by spin coating. Although the rotation rate depends on the viscosity of the slurry, the desired coating thickness and the surface of the substrate, a rotation rate of 1000 and 2000rpm was found to be sufficient. A range of substrates can be used including metals, glass, silicon, ceramics, polymers and composites. However, the matrix should be fairly flat.

● the coated substrate was placed in an oven at a temperature of about 400 deg.C for 10-30 minutes.

Fig. 1 and 2 show scanning electron micrographs of the surface of a hydrophobic coating prepared by the above technique. The surface is magnified 5,000 times (fig. 1) and 100,000 times (fig. 2). It is clear from the photograph that the coating has a very rough surface. Furthermore, the dimensions of the surface roughness of spin coating are at least three orders of magnitude in the size of tens of nanometers to tens of micrometers.

Example 2

In this comparative example, example 1 was repeated except that the amount of Aerosil200 was changed to 1.5 g. As the PDMS/Aerosil ratio increases, the bonding force between the silica particles increases without decreasing the hydrophobicity.

Example 3

In this comparative example, example 1 was repeated except that the amount of TEOS was reduced from 5g to 2.5 g. It was found that the hydrophobicity of the coating did not decrease with the decrease of TEOS as a cross-linker. The flexibility of the coating should increase with a corresponding increase in PDMS in the coating. However, the durability of the coating may be reduced due to the reduced bonding forces between the silica particles and between the particles and the substrate.

Example 4

In this comparative example, example 2 was repeated except that the amount of TEOS was reduced from 5g to 2.5. It was found that with the decrease of the TEOS/Aerosil ratio and the PDMS/Aerosil ratio, the final coating had good durability and flexibility.

Example 5

In a comparative example, example 1 was repeated except that the viscosity of PDMS was increased from 90 to 150cSt to 2,000 cSt. As the PDMS chains in the coating grow longer, the flexibility of the coating should increase.

Example 6

In this comparative example, example 2 was repeated except that the viscosity of PDMS was increased from 90 to 150cSt to 2,000 cSt.

Example 7

In this comparative example, example 3 was repeated except that the viscosity of PDMS was increased from 90 to 150cSt to 2,000 cSt.

Example 8

In this comparative example, example 4 was repeated except that the viscosity of PDMS was increased from 90 to 150cSt to 2,000 cSt.

Example 9

In this comparative example, example 1 was repeated except that the viscosity of PDMS was increased from 90 to 150cSt to 20,000cSt or more. As the PDMS chains in the coating grow further, the flexibility of the coating should increase further.

Example 10

In this comparative example, example 2 was repeated except that the viscosity of the PDMS used was increased to 20,000 cSt.

Example 11

In this comparative example, example 3 was repeated except that the viscosity of the PDMS used was increased to 20,000 cSt.

Example 12

In this comparative example, example 4 was repeated except that the viscosity of the PDMS used was increased to 20,000 cSt.

Example 13

In this comparative example, example 1 was repeated except that Aerosil200 was replaced by Aerosil 8202 (also available from degussa limited, average particle size of 17 nm). It was found that with these particles, the hydrophobicity of the resulting coating was not substantially reduced.

Example 14

In this comparative example, example 1 was repeated except that Aerosil200 was replaced with tin oxide powder having an average particle size of about 300-500 nm. It was found that with a significant increase in particle size, the hydrophobicity of the coating decreased, but not substantially. This may indicate that hydrophobicity is affected by surface roughness and to a lesser extent by particle size.

Example 15

In this comparative example, example 1 was repeated except that Aerosil200 was replaced by antimony (III) oxide powder having an average particle size of about 5,000 nm. It was found that this significant increase in particle size substantially reduced the hydrophobicity of the coating.

Table 1 below shows the contact angles tested on the coatings prepared in examples 1-14.

Example 16

In this example, a hydrophobic coating of silica was prepared using hexane instead of ethanol and including a surface modifier.

A mixture containing 1.5g tetraethyl orthosilicate (TEOS), 150mg water (acidified to pH 4 with HCl), and 50ml hexane was refluxed for 4 hours. The following were added to the reaction system: 2g of PDMS end-capped with hydroxyl groups (viscosity changed to 50,000cSt), 0.5g of ordinary Aerosil silica powder (not subjected to hydrophobic treatment), 1.5g of methyltriacetoxysilane and about 50ml hexane. The reactants were mixed thoroughly and vibrated in an ultrasonic bath at a frequency of about 40kHz for 15-30 minutes to disperse the particles and form a homogeneous slurry. The reaction mixture was removed from the ultrasonic bath and the resulting slurry was sprayed to form a hydrophobic coating. During the spraying process, the slurry is sprayed onto the substrate at the appropriate distance, flow rate, and time to form a uniform coating. After coating the substrate with the slurry, the hydrophobic coating thus formed is heat treated to remove residual solvent. Curing for 10-30 minutes in a curing furnace at the temperature of 120-400 ℃. The contact angle of the coating was 155 °.

TABLE 1

EXAMPLES numbering contact angles (°)

1 167

2 164

3 167

4 153

5 158

6 174

7 169

8 145

9 164

10 158

11 172

12 170

13 159

14 151

15 115

16 155

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A method of preparing a substance capable of being applied to a surface comprising the steps of:

(a) providing a precursor capable of reacting to form a gel,

(b) the precursors are reacted together to form a gel,

(d) treating the mixture to form a modified gel, wherein the particulate matter is bound to the gel and the modified gel is capable of forming a hydrophobic surface having a water contact angle of at least150 °.

2. A method according to claim 1, wherein the modified gel is capable of forming a hydrophobic surface having a contact angle of at least 155 °.

3. A method according to claim 1 or 2, wherein the modified gel is capable of forming a hydrophobic surface on which the contact angle is at least 160 °.

4. A method according to any one of the preceding claims wherein the modified gel is capable of forming a hydrophobic surface on which the contact angle is at least 165 °.

5. The process according to any one of the preceding claims, wherein the precursor provided in step (a) comprises at least water, a solvent and a metal alkoxide.

6. The method of claim 5 wherein the solvent comprises an alcohol.

7. The process according to claim 6, wherein the alcohol is selected from the group consisting of methanol, ethanol, isopropanol and butanol.

8. The process according to claim 5, wherein the solvent is selected from hexane and diethyl ether.

9. The process according to any one of claims 5 to 8, wherein the metal alkoxide is selected from tetramethoxysilane, tetraethoxysilane, tetraisopropoxytitanium, tetramethoxytitanium, tetraethoxytitanium, tetrabutoxytitanium, n-butoxyzirconium.

10. A method according to any one of the preceding claims wherein step (b) of reacting the precursors together comprisesrefluxing the precursors for an extended period of time.

11. A method according to any one of the preceding claims wherein the particulate material comprises particles of substantially the same diameter.

12. A method according to any one of claims 1 to 10 wherein the particulate material comprises particles having a range of diameters.

13. A method according to claim 11 or 12 wherein at least some of the particles have a diameter of

14. A method according to claim 11 or 12 wherein at least some of the particles have a diameter in the range 1nm to 100 nm.

15. A method according to claim 11, wherein substantially all of the particles have a diameter in the range 1nm to 500 μm.

16. A method according to claim 11 or 12, wherein the particles have a primary particle diameter in the range 5nm to 50 nm.

17. A method according to claim 11 or 12, wherein the particles have an average particle size of from 5nm to 20 nm.

18. A method according to claim 11 or 12, wherein the average particle size is about 15 nm.

19. A method according to any one of the preceding claims, wherein the method further comprises a further step prior to step (d), the step comprising mixing a polymer component into the gel, wherein the polymer component is capable of combining with the gel and the particulate material during step (d).

20. A method according to claim 19, wherein the polymeric material is hydrophobic or rendered hydrophobic during step (d).

21. A method according to any preceding claim, wherein the method further comprises, prior to step (d), a further step which comprises adding a surface modifying agent to the gel, the surface modifying agent being capable of increasing the inherent chemical hydrophobicity of the hydrophobic surface formed by the modified gel.

22. The method of claim 22, wherein the surface modifying agent further enhances the bonding between the particulate matter and the gel.

23. A method according to claim 21 or 22, wherein the surface modifying agent is a compound containing one or more hydrophobic groups and one or more condensation curable groups.

24. The method of claim 23, wherein the one or more hydrophobic groups comprise one or more of methyl, ethyl, vinyl, trifluoropropyl.

25. A method according to claim 23 or 24, wherein the one or more condensation cure groups comprise one or more of acetoxy, enoyloxy (enoxy), oxime, alkoxy or amine.

26. A process according to any preceding claim, wherein the particulate material comprises flame-hydrolysed silica powder and the gel comprises silica gel.

27. The method according to any one of the preceding claims in combination with claim 19, wherein the polymer component comprises Polydimethylsiloxane (PDMS).

28. A method of forming a coating on a substrate comprising the steps of:

-forming a modified gel according to the method of any one of the preceding claims;

-coating the modified gel onto a substrate, and

-treating the coated modified gel to form a coating on the substrate, the coating having a hydrophobic surface with a water contact angle of at least 150 °.

29. The method of claim 28, wherein the hydrophobic surface of the coating is a hydrophobic surface having a water contact angle of at least 155 °.

30. The method of claim 28, wherein the hydrophobic surface is a hydrophobic surface having a water contact angle of at least 160 °.

31. The method of claim 28, wherein the hydrophobic surface is a hydrophobic surface having a water contact angle of at least 165 °.

32. The method of any one of claims 1 to 31 wherein the modified gel is in the form of a slurry.

33. The method according to any one of claims 28 to 32, wherein the step of coating the modified gel onto the substrate comprises using the following technique: spin coating, dip coating or spray coating.

34. The method according to any one of claims 28 to 33, wherein the step of treating the mixture comprises drying the applied modified gel to form a solid coating.

35. The method according to claim 34, wherein the drying step comprises heating the coated modified gel to a temperature sufficient to evaporate any solvent.

36. The method as set forth in claim 34 or 35 wherein the step of drying the coating comprises heating the coating to a temperature of 120 ℃ to 400 ℃.

37. A modified gel prepared according to the method of any one of claims 1-27.

38. An object having a surface, wherein at least a portion of said surface is coated with a hydrophobic coating formed from the modified gel prepared by the method of any one of claims 1-27.

39. A hydrophobic coating prepared according to the method of any one of claims 28-36.

40. An object having a surface, wherein at least a portion of said surface is coated with a hydrophobic coating prepared by the method of claims 28-36.

41. A method of preparing a substance capable of being applied to a surface substantially as herein described with reference to the examples and drawings.

42. A method of forming a coating on a substrate substantially as herein described with reference to the examples and drawings.